This
study concerns clay/polymer composites in which nanometer-thick clay sheets
("exfoliated clay") get randomly distributed in a polymer matrix, with random
orientation of the clay sheets. The focus of the study is on a model for the exfoliation
mechanism that describes a recently discovered phenomenon where exfoliation
occurs nearly instantaneously (10 minutes at suitable conditions). Surprising
is that the self-exfoliation is so fast and that it occurs without the help of stirring
or sonication, just by itself after having gently distributed clay particles in
the polymer. Unclear is the underlying mechanism that causes the rapid
self-exfoliation. Knowledge of the mechanism will allow optimal processing of
layered materials, not only clay but also graphite. Such knowledge is important
since a suitable arrangement of nm-thick solid sheets in a nanocomposite can result
in advantageous properties such as increased heat resistance, low gas
permeability, favorable mechanical properties over pure polymer components,
increased electrical conductivity, and light weight compared to conventionally
filled polymers. Here, we propose a novel mechanism, check its consistency with
known observations on nanocomposites, and select an experiment to further test
the proposed mechanism.

This
experimental study is designed to focuses on nanocomposites with layered
silicate (stack of nm-thick organo-clay sheets), such as montmorillonite, as
the reinforcing component of a polymer (end-functionalized polybutadiene). The experiments are guided by a newly proposed exfoliation
mechanism which consists of three steps. Novel is especially the first step:
the proposed pulling of anchored, telechelic ("sticky") macromolecules on the
outer surface of clay particles. The macromolecules form a polymer brush. The
second step is the swelling of the clay particles to a new quasi-equilibrium
state in which the outer brush force, due to the tethered macromolecules, is
balanced by the inner cohesion forces between the sheets in the stack. The
third and final step is diffusion and pressure driven flow of matrix molecules
into the expanded clay galleries to the point that they fully exfoliate.
The model can especially explain the nearly instantaneous exfoliation since
little time is required for brush absorption and expansion of the clay stacks
and the establishing of the force balance. The external brush will form quickly
and its entropic force gets instantaneously balanced by swelling the stack to
raise the internal cohesion force. The rapid expansion of the clay stacks is
followed by a slow approach of the final state by molecular diffusion. It is a
two-tier process which became visible in the time-resolved rheology data of
this study.

The
newly proposed exfoliation mechanism is consistent with the exfoliation
dynamics as observed through rheological observations. It also predicts a
maximum exfoliation rate at an intermediate temperature. A test of this
prediction is at the core of first year's research: The anchored polymer chains
are in constant thermal motion. At higher temperatures the entropic pulling
force increases but macromolecules have less probability to remain absorbed on
the clay surface as the hydrogen bonds are overcome by the increasing thermal
energy. These competing phenomena can be expected to increase the entropic
pulling of the brush at a moderate temperature rise but will eventually break
down the entopic pulling action since fewer and fewer surface molecules will be
able to anchor. Fastest exfoliation is expected at an intermediate temperature.
Experiments of this study confirm the predicted temperature dependence. The
exfoliation rate increases when raising the temperature by a reasonable amount.
However, beyond a certain temperature, the entropic pulling force weakens due
to decreased surface coverage (diminished brush).

A research paper is
about to be submitted to Macromolecules, an ACS journal.

A
related study focused on the relaxation dynamics of amorphous materials, which
are in the approach a liquid-to-solid transition from the liquid side (LSTLS).
Linear viscoelastic experiments on two representative materials focus on the
distribution of relaxation modes and the increasing elasticity in the LSTLS
approach. The first material is a concentrated colloidal suspension; it
represents the glass transition. The second material is a crosslinking polymer
far above its glass transition; it represents gelation. For both, the
relaxation time spectrum broadens significantly near LSTLS and was found to
share the same powerlaw format. A distinctive difference comes from the
powerlaw exponent, n, which is positive for the glass transition,
n>0, and negative for gelation (-1<n<0). The spectrum, H=
0 , is cut off above the longest relaxation time, which belongs to the
diverging, largest cluster in the glass or gel. The front factor, H0,
determines the basic stress level in a relaxation process. Stress under
deformation is governed by a wide range of relaxation modes; as argued here,
short modes overpower the long modes in gelation and long modes dominate over
short ones in the glass transition, i.e. the relaxation patterns are inverse
with respect to each other. Several examples are shown for each class of
materials in order to test the proposed transition behavior for glasses
(colloidal and molecular) on the one hand and chemical/physical gels on the
other. Among several results, this experimental study provides a decisive
criterion that distinguishes the glass transition from gelation.

The
study was submitted to "Soft Matter" under the title: The Glass Transition
as Rheological Inverse of Gelation. The
review process is in progress.